This invention relates to a furnace having a walled enclosure defining a heating chamber which is adapted to be heated by at least one burner supported by the enclosure and operable to direct a flame into the chamber. A typical burner includes a tubular metallic body adapted to mix and ignite a flow of fuel and combustion air to produce the flame. For the most part, the flame is initially produced within a tubular burner block made of ceramic or other non-metallic refractory material and attached to the downstream end of the burner body. The burner block is capable of withstanding extremely high temperatures and usually is formed with a constricted nozzle for shaping the flame and increasing the velocity thereof.
All of the heat produced in the heating chamber cannot be conserved and used and thus a substantial amount of heat escapes from the chamber through the walls of the furnace and via the products of combustion or flue gas expelled from the furnace. It has been found advantageous to utilize this escaping heat to raise the temperature of the air which is supplied to the burner. One arrangement for achieving this is disclosed in Twine U.S. Pat. No. 3,583,691 wherein heat escaping from the furnace chamber is used to preheat the combustion air supplied to the burner.